Among photosensitive resin compositions, ones to which photolithography is applicable are called photoresists and widely used in semiconductors, MEMS/micromachine applications, and the like. In such applications, photolithography is performed through the successive steps of pattern exposing on a substrate and developing in a developer for selective removal of the exposed areas or the non-exposed areas. The photoresists have a positive type and a negative type. A positive type allows the exposed portions to be dissolved in a developer. In contrast, the negative type allows the exposed portions to be insolubilized. In an advanced technology field, electropackage applications and MEMS applications require not only the capability of forming a uniform spin coating film, but also a high aspect ratio, the vertical sidewall profile of a thick film, high tight adhesion to a substrate, and the like. The aspect ratio is calculated from (resist film thickness)/(pattern line width), indicating the performance of photolithography as one of the important properties.
According to the composition having the main component of a bisphenol A type novolac epoxy resin disclosed in Patent Literature 1 and Non Patent Literature 1, a photosensitive image having an extremely high resolution and a high aspect ratio, and a cured product of photosensitive resin can be formed. However, the produced cured resin product tends to be weak for a physical stress depending on application, occasionally resulting in cracking (crazing) during development or when internal stress is generated. Accordingly, the cured resin product may cause not only reduction in adhesion depending on the type of substrate to which the resin composition is applied, but also peeling of the cured product from a substrate in some cases. All the problems occur during cure shrinkage of the composition due to the stress accumulated in the cured resin product. Large cure shrinkage causes bending (warpage) of a substrate in many cases.
Further, it was found that a substrate with the cured resin product formed thereon caused peeling of the cured resin product from the substrate when subjected to a pressure cooker test (PCT), i.e. an accelerated durability test (at 121° C., 100%, 50 hours). Accordingly, use of the cured resin product in applications in an MEMS package, a semiconductor package, a component for forming microreactors, and the like caused inconvenience with poor durability.
Patent Literature 2 discloses a photosensitive composition comprising an epoxy resin having an average number of functional groups of 1.5 or more, a hydroxyl-containing additive, and a cationic photopolymerization initiator. The Literature describes that addition of the hydroxyl-containing additive increases the flexibility of a coating film with a thickness up to 100 μm with the effect of reducing shrinkage. The composition disclosed in the Literature, however, has a resolution as a photoresist of about 90 μm, producing a photosensitive image with residues of skirt shape on the edge portions and a round shape on the top portion after development. As a result, the photosensitive composition was significantly unsuitable for an MEMS package, a semiconductor package, a microreactor and the like.